This invention relates to apparatus for measuring the diffusive resistance of plant stomata.
Measurement of the diffusive resistance of plant stomata is one of the most significant measurements in the determination of water loss by plants. Its measurement is therefore important in the fields of plant science and microclimatology.
The flux of water vapour from a plant leaf is determined by the water vapour concentration gradient between the substomatal cavity and the ambient air and the diffusive resistance in the flow path. The diffusive resistance is the sum of the boundary layer resistance and stomatal resistance.
A known apparatus for measuring diffusive resistance of plant stomata operates by maintaining the boundary layer resistance at a constant value, thus enabling the stomatal resistance to be measured directly. The known apparatus of this kind comprises a cup adapted to be sealingly clamped to a plant leaf being tested to define an enclosed air volume in the cup adjacent to the surface of the leaf. The cup contains a humidity sensor to sense the humidity of the air within the cup and a fan is connected to the cup to enable dry air to be blown into the cup.
The loss of water vapour by the leaf during a measurement cycle increases the humidity of the air within the cup at a rate which depends upon constant characteristics of the cup and upon the diffusive resistance of the leaf. The diffusive resistance is therefore directly related to the rate of increase of humidity of the air within the cup. By measuring the time taken for the humidity to rise between a lower and an upper set value, the value of the diffusive resistance can be found by comparison with an empirical calibration obtained using perforated plates of known diffusive resistance. This measurement is repeated in consecutive measurement cycles, the fan being operated to reduce the humidity of the air within the cup after each measurement cycle in preparation for the next cycle.
In the known apparatus the two set values of humidity are fixed values, for example 60% and 70% relative humidity respectively, and the fan is operated after each measurement cycle to dry the air within the cup down to a humidity lower than the lower set values, i.e. down to a value of 25% relative humidity in the example given. The use of fixed set values of humidity means that the humidity conditions under which the diffusive resistance is measured may differ considerably from the conditions to which the leaf was subjected in the ambient air immediately prior to testing. Moreover, the large repetitive drying range may produce a compensating reaction in the leaf stomata and thus give an erroneous reading.
It is an object of the present invention to provide an improved apparatus which enables the diffusion resistance of plant stomata to be measured more accurately and in conditions which conform more closely to the ambient conditions of the plant before measurement.